Electric power generators are devices that convert mechanical energy into electrical energy. In an AC generator, the rotor is driven by e.g. a turbine and electric currents are induced in the stator windings of the generator. An AC generator is normally either of a synchronous type or an asynchronous type. For a long time synchronous generators have been used in power systems of different kinds. Synchronous generators are e.g. used both in large power systems, such as nuclear power plants, as well as in small isolated systems, such as wind power plants.
In contrast to the to the asynchronous generator (or induction generator) which uses induction to provide a magnetic flux, the synchronous generator uses either a permanent magnet (PM) or electrical magnetization to produce its magnetization flux. In this respect, the use of permanent magnet generators has found increasing acceptance in recent years, mainly due to the need for inexpensive and reliable excitation components. The application of new permanent magnetic materials has resulted in high-efficiency generators which are superior to other types of generators in many ways.
A synchronous generator normally comprises three phases, but in recent years many investigations related to multiphase generators have been made, a lot of them towards six phase generators. The interest in multiphase generators lies mainly in the fact that with many phases the high currents associated with high power generators can be divided among more phases. Other advantages of multiple phase generators compared to three phase generators are e.g. lower noise levels at the same power level, lower harmonic distortion and lower EMI.
Generators with six or more phase windings are mainly found in full rate wind turbine power conversion application where load sharing and output power distribution among the converter modules as well as output power scalability in low and high wind are desired. One approach to implementation of a multiphase generator, such as a six phase generator, is to use a modular generator, i.e. a generator composed of multiple three phase generators with interleaved windings as illustrated in FIG. 1a with a six-phase 8-pole PM generator.
The advantages of the modular generator with interleaved winding configuration include (1) The mechanical force is always balanced with respect to the generator shaft and the mechanical stress is evenly distributed over the entire generator structure irrespective of the output power level of individual three phase systems; (2) Identical reluctance in the air gap for each three phase systems, which facilitates identification and measurement of the generator parameters in one set of three phase system for control purpose; (3) Failure of any sub-system does not require the entire power generation system to be shutdown. The reliability and availability of power output is thus improved.
Compared to the operation of a single three phase generator, when operating a modular generator comprising e.g. two or more three phase interleaved windings it is important to take into consideration the dynamic load or power sharing of the different three phase systems. More specifically, for the modular PM generator composed of multiple three phase generators with interleaved windings, a conventional current feedback field oriented control is not sufficient due to the significant magnetic coupling between each three phase system.
The normal way to design a six phase modular generator is to arrange two star connected windings with a 30 electrical degrees phase-shift between the two stars. By this arrangement the 11th and 13th harmonics will be reduced thereby lowering the stress on the generator system. Another way is to split the phase belt of a conventional three-phase machine into two parts with spatial phase separation of 30 electrical degrees. Yet another way is to use a machine with star connected windings and arranging a star-triangle transformer at the output of one generator in order to get a 30 electrical degrees phase-shift between the two generators. However, the first described design is the most common arrangement today.